二叉树的深度

递归(深度遍历dfs)
时间复杂度:O(n),遍历二叉树每个结点
空间复杂度:O(n),递归栈深度就是二叉树的高度,其中最坏情况是二叉树退化为链表,深度最大为n

/*
struct TreeNode {
	int val;
	struct TreeNode *left;
	struct TreeNode *right;
	TreeNode(int x) :
			val(x), left(NULL), right(NULL) {
	}
};*/
class Solution {
public:
    int TreeDepth(TreeNode* pRoot) {
        if(pRoot==nullptr) return 0;
        int lDepth = TreeDepth(pRoot->left);
        int rightDepth = TreeDepth(pRoot->right);
        return max(lDepth, rightDepth) + 1;
    
    }
};

class Solution {
public:
    int TreeDepth(TreeNode* pRoot) {
        if(pRoot==nullptr) return 0;
        return max(TreeDepth(pRoot->left), TreeDepth(pRoot->right)) + 1;
    }
};

层次遍历(dfs)
时间复杂度:O(n),遍历二叉树每个结点
空间复杂度:O(n),辅助队列的最大长度不会超过n

class Solution {
public:
    int TreeDepth(TreeNode* pRoot) {
        if(pRoot==nullptr) return 0;
        queue<TreeNode*> q;
        q.push(pRoot);
        int level = 0;
        while(!q.empty()){
            int size = q.size();
            while(size--){
                auto node = q.front();
                q.pop();
                if(node->left) q.push(node->left);
                if(node->right) q.push(node->right);
            }
            level++;
        }
         return level;
    }
};
class Solution {
public:
    int TreeDepth(TreeNode* pRoot) {
        if(pRoot==nullptr) return 0; //空结点没有深度
        queue<TreeNode*> q; // 生成队列
        q.push(pRoot); //根入队
        int level = 0; //记录深度 
        while(!q.empty()){ // bfs
            int size = q.size(); //记录当前层有多少结点
            for (int i=0; i<size; i++) {
                auto node = q.front(); // TreeNode* node = q.front(); 
                q.pop();
                if(node->left) q.push(node->left);
                if(node->right) q.push(node->right);
            }
            level++; // 每访问完一层结点,深度加1
        }
         return level;
    }
};
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